FIG. 1 shows a format of a sector on a magnetic disk processed in a conventional magnetic disk. In FIG. 1, numeral 1 denotes a preamble pattern for clock synchronization, 2 a decoding start synchronization mark, 3 a data area and 4 error correction parity bytes. In the data area 3 and the error correction parity bytes 4, a decoding method such as (1,7) RLLC (Run Length Limited Code) or (2, 7) RLLC is used to record digital data.
FIG. 2 illustrates a configuration of a conventional magnetic disk drive apparatus. In FIG. 2, numeral 5 denotes a clock synchronization circuit, 6 a decoding start synchronization mark detection circuit, 7 a decoding circuit, and 8 an error correction circuit.
Operation of the conventional magnetic disk drive apparatus is now described. A binarized reproduction pulse a of a reproduced signal is supplied to the clock synchronization circuit 5. The clock synchronization circuit 5 establishes clock synchronization by using the clock synchronization preamble pattern 1 and produces an encoded data b. The encoded data b is supplied to the decoding start synchronization detection circuit 6 and the decoding circuit 7. When the decoding start synchronization mark detection circuit 6 detects the decoding start synchronization mark 2, the detection circuit 6 supplies a decoding start command c to the decoding circuit 7 to start decoding. The decoding circuit 7 decodes data in the data area 3 and the error correction parity bytes 4 to data in the form of NRZ (Non-Return-to-Zero) to produce decoded data d, which is supplied to the error correction circuit 8. The error correction circuit 8 corrects error in the decoded data d by means of the error correction parity bytes 4, if the decoded data contains any error, and produces reproduced data e of the data area 3.
When the clock synchronization preamble pattern 1, the decoding start synchronization mark 2, the data area 3 and the error correction parity bytes 4 have 12, 4, 512 and 32 bytes, respectively, and if an error correction code format shown in FIG. 3 is used, the error correction circuit 8 has the correction capability with respect to random error that a probability A of producing a sector having one or more errors can be improved to a probability B of producing a sector having one or more uncorrectable errors as shown in FIG. 4. Further, burst errors continuing up to 16 bytes can be corrected.
In the conventional magnetic disk drive apparatus, however, as shown in FIG. 4, a probability C of producing a sector in which the decoding start synchronization mark 2 can not be detected is larger than the probability B of producing a sector having one or more uncorrectable errors in an area of the production probability of 10.sup.-3 or less. Accordingly, since the decoding circuit 7 can not be operated before the error correction circuit 8 is operated, there is a problem that an actual probability of producing failed sectors is as shown by a broken line D and the random error correction capability of the error correction circuit 8 can not be utilized sufficiently.
Further, even if the error byte number is one byte, the error can not be corrected when the error is produced in the decoding start synchronization mark 2 and accordingly there is a problem that the burst error correction capability of the error correction circuit 8 capable of correcting the burst error having up to 16 bytes can not be utilized sufficiently.